Ggerg1186

11-24-2012, 09:27 AM

So my son's cub scout leader asked me to talk to the Webelo Scouts (4-5 graders) about engineering. So I rigged up a crane with my old lego set to demonstrate distance, force, work, and power. I can hang 1-6 weights on the hook and shift gears between ...1:1, 1:3, and 1:6. in first gear (1:1) it takes 2 seconds to lift 1 weight 6 ft. This is 6 units of work. In second gear it takes 6 seconds to lift 3 weights 6 ft. This is 18 units of work. In third gear it takes 12 seconds to lift 6 plates 6 ft. This is 36 units of work. Power is work divided by time, so the power for all three runs is 3 units of power. I was pretty happy when my stop watch agreed with my initial estimate.

I would like your critiques and ideas for my presentation....

http://i5.photobucket.com/albums/y191/Ggerg1186/LegoCrane.jpg

Now I understand that the speed is slower because the gear ratio is lower. However, I wanted to find a way to demonstrate power without getting into the details of motor speed/torque-efficiency curves. A key concept for me is that power remains the same for all runs.

I plan to spend 10 minutes or so talking about engineering and engineering education. "No, you don't have to be good at math, but you do have to be willing to spend a lot of time learning math." "It helps to be curious about technical things" etc.

For the presentation I plan to show the crane & weights and ask them to predict how much power then motor can produce. (by comparision)..."is this motor as powerful as the one in your parent's car? How about some of your toys? Do you think this lego motor could replace the motor in my 6,000 lb truck?

I would expect the answer to the last question will be no. I will then say, let's find out. First, we need a unit of time (ask scouts for examples of time)...anyone have a stopwatch on their wristwatch/phone? Plan to use seconds.

We need a unit of distance, Question: what do we use to measure distance? (answers, miles feet inches, etc) Explain that the unit of feet was once measured by the length of a king's foot. Let's use that, who wants to volunteer? Take long piece of paper and let one student use the length of his shoe to measure approx 8 evenly spaced lengths on the paper. What should we call this? How about a tiger paw? (tiger scout is the youngest cub scout) Hang the scale on the wall next to bookshelf in room.

We need wieght, I have six evenly sized steel plates, how much do you think they weigh? guesses...I do not know how much it weighs, but I think they all weigh the same, so lets make our own unit of weight, what do you want to name the weight for each plate? (something cub scout related? - bobcat)

Question: what is work? If someone writes a one page paper and someone else writes a two page paper for school, who did more work? If you carry one bag of groceries in from the car while your mother carries 4, who did more work? How much more work? Which is more work for the crane, one plate or six? What are the units for work (I don expect many answers about joules or BTU's) Lets call one tiger paw times one bobcat one unit of work, and lets call that a Webelos. So if the crane lifts two bocat plates 2 tiger paws we have done 4 units of work, 4 Webelos...

Start experiment:

Have a scout run a stop watch. Give him a few times to practice. Put crane in 1st gear and hang one weight. Measure time to lift 8 tiger paws. Have scouts calculate and record time, distance, weight, and work. Place three weights on crane, ask scouts to predict how long it will take and record guesses. Start crane, crane stalls...uh oh, now what? Anyone have a bicycle that can shift gears? what do you do when it gets hard to pedal? Shift crane to second gear and run experiment, have scout record time, distance, weight, and work. Place six plates on crane, ask scouts to predict time required. (I think that they will predict six times as long after the second run is 3 times as long with three plates)

Ask them if we can prove it with math. Introduce the concept of power...power is the rate of work being done. If you write a one page paper in an hour and your friend writes a two page paper in 3 hours, who did more work, who worked faster? Who was more "powerful?"

Explain that power is work completed divided by the time to complete that work. Let's create a name for this new unit...(scouts create a name such as cubs) One cub is one Webelos divided by one second. Look at data, have scouts calculate power for run #1 and #2. Should come up with a similar number for both runs. Use that number to confirm guesses for lifting 6 plates. Start crane, crane stalls... shift to gear six by rigging up 2:1 block and tackle, talk about block and tackle from Webelos book. Run #3 and document time, distance, weight, work, and power. Confirm prediction, explain why it may not be exactly the same (battery life, efficiency, etc) Discuss how the motor could lift anything with the proper gear ratio (not entirely true, but forget efficiency for now)

Ask them "Can this lego motor replace the engine in my 6,000lb truck?" The answer is yes, but I would have to go very slow.

What do you think? Any additions/deletions?

I would like your critiques and ideas for my presentation....

http://i5.photobucket.com/albums/y191/Ggerg1186/LegoCrane.jpg

Now I understand that the speed is slower because the gear ratio is lower. However, I wanted to find a way to demonstrate power without getting into the details of motor speed/torque-efficiency curves. A key concept for me is that power remains the same for all runs.

I plan to spend 10 minutes or so talking about engineering and engineering education. "No, you don't have to be good at math, but you do have to be willing to spend a lot of time learning math." "It helps to be curious about technical things" etc.

For the presentation I plan to show the crane & weights and ask them to predict how much power then motor can produce. (by comparision)..."is this motor as powerful as the one in your parent's car? How about some of your toys? Do you think this lego motor could replace the motor in my 6,000 lb truck?

I would expect the answer to the last question will be no. I will then say, let's find out. First, we need a unit of time (ask scouts for examples of time)...anyone have a stopwatch on their wristwatch/phone? Plan to use seconds.

We need a unit of distance, Question: what do we use to measure distance? (answers, miles feet inches, etc) Explain that the unit of feet was once measured by the length of a king's foot. Let's use that, who wants to volunteer? Take long piece of paper and let one student use the length of his shoe to measure approx 8 evenly spaced lengths on the paper. What should we call this? How about a tiger paw? (tiger scout is the youngest cub scout) Hang the scale on the wall next to bookshelf in room.

We need wieght, I have six evenly sized steel plates, how much do you think they weigh? guesses...I do not know how much it weighs, but I think they all weigh the same, so lets make our own unit of weight, what do you want to name the weight for each plate? (something cub scout related? - bobcat)

Question: what is work? If someone writes a one page paper and someone else writes a two page paper for school, who did more work? If you carry one bag of groceries in from the car while your mother carries 4, who did more work? How much more work? Which is more work for the crane, one plate or six? What are the units for work (I don expect many answers about joules or BTU's) Lets call one tiger paw times one bobcat one unit of work, and lets call that a Webelos. So if the crane lifts two bocat plates 2 tiger paws we have done 4 units of work, 4 Webelos...

Start experiment:

Have a scout run a stop watch. Give him a few times to practice. Put crane in 1st gear and hang one weight. Measure time to lift 8 tiger paws. Have scouts calculate and record time, distance, weight, and work. Place three weights on crane, ask scouts to predict how long it will take and record guesses. Start crane, crane stalls...uh oh, now what? Anyone have a bicycle that can shift gears? what do you do when it gets hard to pedal? Shift crane to second gear and run experiment, have scout record time, distance, weight, and work. Place six plates on crane, ask scouts to predict time required. (I think that they will predict six times as long after the second run is 3 times as long with three plates)

Ask them if we can prove it with math. Introduce the concept of power...power is the rate of work being done. If you write a one page paper in an hour and your friend writes a two page paper in 3 hours, who did more work, who worked faster? Who was more "powerful?"

Explain that power is work completed divided by the time to complete that work. Let's create a name for this new unit...(scouts create a name such as cubs) One cub is one Webelos divided by one second. Look at data, have scouts calculate power for run #1 and #2. Should come up with a similar number for both runs. Use that number to confirm guesses for lifting 6 plates. Start crane, crane stalls... shift to gear six by rigging up 2:1 block and tackle, talk about block and tackle from Webelos book. Run #3 and document time, distance, weight, work, and power. Confirm prediction, explain why it may not be exactly the same (battery life, efficiency, etc) Discuss how the motor could lift anything with the proper gear ratio (not entirely true, but forget efficiency for now)

Ask them "Can this lego motor replace the engine in my 6,000lb truck?" The answer is yes, but I would have to go very slow.

What do you think? Any additions/deletions?